A comparative computational and scanning electrochemical microscopy study of factors influencing electron transfer at the hydrogenated and pristine graphite - propylene carbonate electrochemical interface

Nonaqueous redox flow batteries are a promising technology that utilize redox-active species (i.e., redoxmers) in solution to store energy via electron-transfer (ET) reactions with electrodes. However, electron transfer (ET) phenomena at the interface of graphitic electrodes and nonaqueous media are poorly understood, with several non-idealities in the use of conventional models such as the Butler-Volmer model reported. Possibilities for these non-idealities include the adsorption of redox species at the electrode, fundamental ET limitations related to the density of states at the electrode, and the presence of chemical and spatial heterogeneities at the surface of the electrode. To this point, we present a computational and experimental approach to comparatively investigate the ET behavior of two redoxmers, ferrocene (Fc) and 2,3-dimethyl-1,4-dialkoxybenzene (C7) on single layer graphene (SLG), hydrogen-functionalized SLG (H-SLG), and pristine and hydrogen-functionalized graphite electrodes. Scanning electrochemical microscopy (SECM) experiments revealed enhanced ET kinetics for both redoxmers on H-SLG electrodes compared to pristine SLG electrodes, with the degree of functionalization playing a key role in this enhancement. Electrodes such as boron-doped diamond and hydrogenated graphite mirrored these enhancements. Density functional theory (DFT) calculations indicate only small differences in the binding strengths for Fc and C7 redoxmers on SLG and H-SLG surfaces, but Marcus-Hush-Chidsey (MHC) kinetic theory analysis suggests that the density of states (DOS) of the carbon electrode likely plays a crucial role in the observed ET enhancement. These findings refine our initial assumption of binding energy (BE) as a dominant factor for interfacial behavior in the case of Fc and C7 redoxmers. Our findings create new opportunities to explore systems with varying degrees of surface modification to understand and design better redox flow batteries.